The progressive electronification of control tasks in automotive technology has led to the application, in modern motor vehicles, of an ever growing number of electrical and electronic units, such as sensors, actuators and control units for the most various tasks. The expenditure for their wiring is increasing with the number of these units. In order to keep within limits the wiring efforts and the costs connected with them, it has proven expedient that one should no longer connect each individual sensor or each individual actuating mechanism via individual signal lines to a control unit that processes one of the sensing signals of the sensor or supplies actuating signals for the actuating mechanism, but rather that one should connect a plurality of such sensors or actuating mechanisms, in each case via a suitable interface switching operation, to a common bus, on which signals in the form of addressed messages can be transmitted in time division multiplex between the control unit and the diverse interfaces.
Although the use of a bus substantially reduces the wiring effort, this advantage bears the cost that the coordination in time of various sensors and/or actuators is made more difficult. In a bus-supported network, in order to carry out actions coordinated in time of various connected data processing units (by which may be understood especially the combination of a sensor or actuator to its assigned interface switching operation), a command has to be sent to these units before the action that is to be executed, which specifies the action and the point in time of its execution, since, as a result of the time division multiplex operation it cannot be ensured that the bus will be free, at the desired point in time of the action, to send commands for immediate execution of the desired action to the desired data processing units. However, if a command specifying the point in time of an action is sent ahead of time to the data processing units, it is imperative for the coordination in time of the actions to be executed, that the units have a common time standard.
One possibility of establishing such a common time standard for all data processing units is the use of a clock signal, transmitted on the bus, whose periods are also counted by the connected units. However, this solution is unsatisfactory to the extent that it requires its own signal line or a great proportion of the transmission bandwidth of a single bus line, and that counting errors, which may occur at the individual data processing units, based on transmission interference, may only be prevented by costly additional measures.
Another possibility is to equip each data processing unit with its own time signal indicator. However, unavoidable scattering of the operating frequencies of these time signal timing devices may have the result that an incipient synchronicity is lost during the course of time, so that here, too, an exact coordination in time of the actions to be executed by the individual units cannot be guaranteed without any problem.